Originally Posted by Homer J. Fong
The intent is clearly to say that Bruce/Batman has been out of action for eight years. Not five, not seven, eight. That seems - and these are probably weird ways to put it - too easy or simple, though. That he would stop right after the events of the TDK finale, that no major crimes would occur in the intervening eight years, that the Dent Act would come about and do its job so quickly...
I understand exactly what you are saying and you made your points very well.
As you say, when broken completely down, it does not 100% follow. The ending of TDK led us to believe that Batman would continue even though he was to be hunted, but then in TDKR we are presented with a Batman that apparantly finished directly after TDK - and the reason given - because the Dent Act made him null and void. As you say, it's is unlikely such an Act would come in to effect quite that quickly.
Of course, as the audience there is much that we are not privy to that we are to imagine happened off-screen.
The truth is probably somewhere in the middle all of this and I think at some point you have to step back from the fine details and think what the true intention of the Writer/Director really was.
Looking at things now, my opinion is that Bruce stopped being Batman immediately after TDK. This is alluded to quite specifically in TDKR. In TDK organised crime had been seriously disrupted, both by Batman/Gordon but also through the Joker killing two of the major kingpins. The city, as we saw, had also chosen to fight back and there was undoubted momentum to clean up the city.
With the Police handling low level crime, the Dent Act was passed relatively soon afterwards which formalised how they would treat organised crime.
So I suspect that Bruce found himself very quickly in a position where his services were not required.
Given the timespan of at least a year between BB and TDK I think it is reasonable to assume that the cave was nearing completion at the time of TDK. If an explanation as to why it was ultimately completed is required then I would suggest this was because there was always the possibility he may need it again (we see in TDKR that he is waiting for such a moment when the city would need him).